Substrate material choice for microfabrication is a factor used in the design of flexible electronics. Flexible displays are a type of flexible electronics. One of the core components in these devices is the display's backplane, which provides a matrix of addressable thin-film transistor (hereafter “TFT”) devices to control the display's pixels. The use of oxide-based semiconductors is becoming increasingly popular for large-area TFT fabrication. This is due to indium gallium zinc oxide (hereafter “IGZO”) TFTs' favorable electrical characteristics, and the ease of converting existing amorphous silicon manufacturing lines for compatibility with IGZO device fabrication. A limitation seen today in the microfabrication of low-cost and high-yield flexible backplane TFT stacks is the flexible substrate material.
The fabrication of this substrate material is a critical step in order to enable these devices. Slot-die coating is a scalable manufacturing technique used to deposit thin-films of liquid material with a high degree of uniformity. In the case of polymer formulations (as used in photoresist, high-performance coatings, or substrates for microfabrication applications), slot-die coating may provide a low-cost method for the fabrication of large-area thin-films. For high-performance organic films, the typical polymer formulation that allows slot-die coating includes a significant liquid fraction (i.e. the solvent) and a low solid fraction (i.e. the solute). However, this method may lead to non-uniformity of the substrate surface due to the evaporation required for the solid fraction to precipitate and form a continuous film.
Current slot-die coatings for flexible electronics employ the use of solvated substrate materials (e.g., polyamic acid solutions), solvated organic photovoltaic materials (e.g., polyfluorenes, polythiophenes), or solvated organic semiconductors (e.g., polycyclic aromatic hydrocarbons) where a majority of the coating may be evaporated away. However, in these applications a substantial fraction may remain in the solid film after curing and this may cause off-gassing during sensitive processing applications such as high-vacuum deposition or thermal annealing.
Other challenges in flexible display backplane manufacturing may be due at least in part to the photolithographic processes that the substrate of the backplane must withstand. Even at temperatures below second-order transition and phase-change temperatures, existing backplane substrates do not allow for reliable and reproducible alignment over subsequent photolithographic steps requiring thermal cycling or cycling of other environmental stimuli. In addition, creep, fatigue, and micro-to-macro-scale network rearrangement cause problems for existing backplane substrates and render them inadequate at higher temperatures, such as those often required for formation of additional backplane structures on the substrate or further electronics structures on the backplane.
Once the thin film devices are fabricated on the flexible substrate, they must be integrated for optimal performance and feasibility using a hybrid approach. Full device construction is necessary wherein the flexible materials (organic substrates, thin-film electronics, etc.) are interfaced with rigid components (traditional surface-mount components, integrated circuits, etc.). However, this interface is typically poor due to the lack of covalent or metallic bonding between the flexible and rigid components. Additionally, mismatches in the physical properties of the materials such as the coefficient of thermal expansion (hereafter “CTE”) can lead to interfacial stresses which when coupled with a poor adhesion at the flexible-rigid interface, lead to delamination of the devices from the substrate and ultimately device failure. Current methodologies for interfacing these materials involve the careful management of interfacial stresses (e.g., the design and use of low-CTE materials, the engineering of strain tolerant interfacial structures, etc.) or the use of through-substrate vias for interconnects between the organic and inorganic components.
The roughness of the flexible polymer substrate is another important factor for consideration in the preparation of a substrate suitable for the microfabrication thin-film structures. Most polymer processing techniques yield substrates with unsuitable surface roughness (Ra>5 nm) due to either technique-induced roughness (e.g. molding, extruding, etc.) or the nature of the polymer backbone behavior (e.g. semi-crystalline, liquid crystalline, etc.) However, by utilizing monomeric or oligomeric precursor solutions, a film may be deposited via a plurality of methods (e.g. die coating, blade coating, dip coating, etc.) that produce continuous films with low surface roughness (Ra<5 nm) and cure the materials in this final, smooth-surface state.
An additional benefit to utilizing materials without semi-crystalline, liquid crystalline, or other backbones with other elevated degrees of secondary interactions is that the amorphous nature of these polymers leads to substrates with a high degree of optical transparency and minimal optical anisotropy (e.g., refractive index anisotropy, through-thickness retardation, etc.) Such materials have desirable applications in devices that utilize electromagnetic waves in the visible spectrum (visible light) as either an input or output, and minimize the optical loss as well as the power required to power such devices.
Current flexible electronics backplane materials, such as biaxially-oriented polyethylene naphthalate, polycarbonate, and polyimides, suffer from the inability to align multiple masks over thermal cycling, which may limit processing temperatures, size, and/or complexity of electronic structures.